A microporous membrane using a polymer material is widely used for a filter such as an air filter, a bag filter and a filter for liquid filtration, and a separator used for a secondary battery, a capacitor or the like. In general, the microporous membrane has higher strength in comparison with a raw material such as a woven fabric and a nonwoven fabric and is excellent in compactness, and therefore among applications described above, in particular, the microporous membrane is widely used as a microfiltration membrane and an ultrafiltration membrane, and a separator for a lithium-ion secondary battery. Above all, a microporous membrane formed of polyolefin-based resin is excellent in chemical resistance and oxidation resistance, and therefore used most frequently.
When the membrane is used for the lithium-ion secondary battery, in order to prevent a rapid temperature rise due to overcharge or a short circuit in connection with achieving high capacitance, safety is improved by using a microporous membrane having a shut-down function of clogging micropores by melting of a polymer at around 120° C., as in a polyethylene microporous membrane, to increase internal impedance and to inhibit progress of a battery reaction, having a function of preventing a short circuit without causing meltdown even at around 150° C., as in a polypropylene microporous membrane, or having both of the shut-down function and prevention of meltdown by laminating both thereof.
However, when temperature exceeds 160° C. being a melting temperature of polypropylene, prevention of the short circuit caused by meltdown is difficult, and if the short circuit is caused, rapid generation of heat occurs. When the temperature reaches 200° C. or higher, oxygen emitted by thermal decomposition of a cathode vigorously reacts with an organic solvent to cause bursting or ignition of a battery due to thermal runaway. Therefore, various attempts have been proposed so far so that the short circuit due to meltdown can be prevented even at the melting temperature of polypropylene or higher.
As an attempt for improving heat resistance of the microporous membrane, for example, a proposal has been made on a heat-resistant separator having a resin porous film with a thermal shrinkage at 150° C. of 10% or more, and a heat-resistant porous layer formed on a surface of the resin porous film and containing 70% by volume or more of heat-resistant particulates (see Patent literature No. 1, for example). Moreover, a proposal has also been made on a separator for a battery prepared by uniting porous film (A) containing at least an inorganic filler and porous film (B) being a mixture of a polyolefin having a melting point less than 150° C. and a polyolefin having a melting point of 150° C. or higher to adjust thermal shrinkage at 150° C. to be 5% or less (see Patent literature No. 2, for example).
In techniques described in Patent literature Nos. 1 and 2, the shut-down function is maintained, and further shutdown can be maintained even under conditions of a higher temperature. However, when the temperature exceeds 160° C. being the melting temperature of polypropylene, melting shrinkage of a polyolefin layer becomes strong, and therefore prevention of the short circuit between electrodes becomes difficult at around 180° C.
Therefore, a proposal has also been made on a heat-resistant separator prepared by providing, with a porous layer mainly containing an inorganic filler having a heat-resistant temperature of 150° C. or higher, a microporous membrane having a sticking strength of 3 N at largest as produced by a dry uniaxial stretching method according to which resistance to thermal shrinkage is relatively satisfactory among microporous membranes (see Patent literature No. 3, for example), and a proposal has also been made on a heat-resistant separator prepared by providing, with a porous layer mainly containing a needle-shaped filler having a heat-resistant temperature of 150° C. or higher, a microporous membrane produced by the dry uniaxial stretching method in an identical manner (see Patent literature No. 4, for example), or the like.
In techniques described in Patent literature Nos. 3 and 4, while the separators are excellent in shrinkage suppression under a high temperature, a degree of orientation of crystals of a polyolefin on a microporous membrane surface tends to become high due to characteristics of a production method by the stretching method. Therefore, concern exists about reduction of adhesion with the inorganic filler to cause dropout of the inorganic filler during a production step and use.